The preparation of fine particle systems such as solid-in-liquid dispersions can be carried out by a wide variety of processes including grinding, homogenization, and precipitation. Amorphous particle-in-liquid systems are usually prepared by incorporating an amorphous dispersed phase into a liquid continuous phase under high shear mixing or homogenization.
In certain applications, such as with photographic dispersions, crystalline materials such as dye-forming couplers, oxidized developer scavengers, and various dyes are dissolved in organic solvents at high temperatures and emulsified in aqueous gelatin solutions. Submicron amorphous particles in such dispersions are found to be metastable and will eventually recrystallize in the aqueous system unless coated and dried on photographic support, in which state they are stable against recrystallization.
Recrystallization of the dispersed particles prior to coating reduces dispersion efficacy and is generally undesirable. In addition, crystallization of the UV absorber after coating may lead to delamination of layers, haze, reduced maximum density, stain, and sensimetric problems.
U.S. Pat. No. 5,110,717, which is incorporated herein by reference, describes an improved process for making amorphous fine-particle dispersions. The process comprises mechanically grinding a crystalline material to a desired particle size in a liquid that is not a solvent for the crystalline material, heating the crystalline particles dispersed in the liquid to above their melting temperature, and cooling the melted particles in the liquid to form amorphous particles. In preferred forms of the invention discussed in the patent, the crystalline materials are photographically useful materials, such as ultraviolet light absorbers and couplers. The dispersions formed by the process are more storage stable and the particles formed are smaller than those formed in other emulsification processes. Small particle size provides more effective UV control for a given amount of UV absorber and allows the use of less silver and less gelatin in film layer formation. Finer UV absorbing compounds give better images in photographic products, as there is less light scattering and better UV absorption for a given amount of material in the product.
In the process described in U.S. Pat. No. 5,110,717, crystalline material and a nonsolvent liquid are added to a media mill. The media mill operates to reduce the crystalline material to the desired size, after which it is passed through a filter and placed in a mixing vat where the liquid to particle ratio may be adjusted. The nonsolvent for the material of the examples is water. The milling and mixing are carried out at about room temperature. The slurry of particles may be either transferred to storage or directly to a subsurface addition device for combination with a gelatin and water solution in a tank. After mixing crystalline material with the gelatin water solution, it is passed from the tank to an inline heater. At the inline heater, the crystalline material is heated to above its melting temperature, typically 75.degree. to 99.degree. C. After heating the material is immediately cooled in an inline cooling section to 40.degree. C. and then immediately coated.
In the apparatus schematically shown in the patent, microcrystalline materials with melting points below 100.degree. C. are converted to approximately spherical amorphous particles in an aqueous dispersion. However, the apparatus shown could not be used to carry out the otherwise attractive process when the microcrystalline material has a melting point above 100.degree. C. and the nonsolvent carrier is water.
There is thus a need for an apparatus for preparing an amorphous particle dispersion in water when the material from which the amorphous particle is formed melts above 100.degree. C.